This invention relates generally to position/orientation tracking and, in particular, to methods and apparatus for accurate position, orientation and movement tracking within a volume in the presence of electromagnetic distortion and noise.
Existing electromagnetic tracking systems, as well as inertial and combined inertial/optical and optical/magnetic tracers, are sensitive to various kinds of distortion. With respect to electromagnetic trackers, such distortion may arise from eddy currents in metal objects, whereas, in the case of inertial trackers, drift or vibration might be the cause.
Inertial tracking systems, as described in U.S. Pat. No. 5,645,077, requires an additional sensor, or set of sensors, to compensate for drift and movement of a vehicle or aircraft reference frame. Even with these additional sensors, such systems exhibit sensitivity to vibration and temperature instability requiring additional compensation. Inertial tracking systems also experience drift over time periods on the order of minutes to hours.
Combination systems, that is, systems which combine optical and inertial or optical and magnetic sensing, are designed to compensate for distortion by comparison of the data from two different types of sensors. One such system is described in U.S. Pat. No. 5,831,260 to Hansen. These approaches are restricted to applications such as interference associated with night-vision devices, and are still affected by distortion, especially when parasitic illumination or optical noise is present. Another system is described in U.S. Pat. No. 6,148,280 to Kramer. This system employs two different kinds of sensorsxe2x80x94on of them is slow, but accurate, another is fast, but less accurate, e. g. optical and inertial sensors. While such a system allows performing tracking with sufficient accuracy and high update rate, it still has the same problems as one referred to above. The system described in European Patent Application No. EP 1 034 738 Al to Govary employs RF illumination of the probe sensor/transducer that emits ultrasound energy at the frequency responsive to an interaction with the RF electromagnetic field. Detectors in a vicinity of the object measure the energy response and the system utilizes this information to compute position of the probe sensor. Such a system allows to use wireless sensor/transducer but have no tools to compensate for distortion and scattering of RF and ultrasound waves.
In an AC electromagnetic tracking environment, distortion may arise from eddy currents induced in nearby metal objects. These currents may, in turn, generate stray fields, which interfere with the field from the source(s) used for tracking purposes. The system described in U.S. Pat. No. 6,147,480 to Osadchy and Govari utilizes the field form eddy currents generated in a metal tool to determine the position of this tool, given that the field from eddy currents is phase shifted with respect to the field from the source. This system is able to find the position of a metal object, but suffering from the distortion grom the surrounding metal and unable to distinguish uniquely multiple objects present in the volume of interest simultaneously. To compensate for the electromagnetic distortion, one solution involves the use of mapping. With mapping, the electromagnetic field in a volume of interest, as distorted by metal objects, is defined in advance and used to solve for position and orientation. Commonly assigned U.S. patent application Ser. No. 09/215,052 describes such a mapping system.
In commonly assigned U.S. patent application Ser. No. 09/430,978, a system for electromagnetic position and orientation tracking is disclosed wherein distortion parameters are computed using data from witness sensors. Each witness sensor has a fixed position and orientation near or within the volume to account for the distortion. One or more probe sensors are placed on an object (or multiple objects) to be tracked within the volume, and the output of each witness sensor is used to compute the parameters of a non-real effective electromagnetic source. The parameters of the effective source are used as inputs to the computation of position and orientation as measured by each probe sensor, as if the object were in the non-distorted electromagnetic field produced by the effective source.
In commonly assigned U.S. Pat. No. 5,640,170, a source is used to generate a plurality of electromagnetic fields which are distinguishable from one another, and a remote sensor has a plurality of field-sensing elements which sense each of the generated electromagnetic fields. A processor processes the output of the sensor in a remote object position and orientation relative to the source reference coordinate frame. At least one of the field-generating elements of the source has at least one electrically conductive sheet and a planar coil including a plurality of coplanar concentric rings above the conductive sheet. The planar coil is configured in a manner that a signal applied to that coil causes a current density at each ring that is inversely proportional to the square of the radius of that ring. Although the source is considered to be a distortion stable source, the resulting tracker is limited to shielding distortion in only one hemisphere.
A wireless, eyeball motion tracking system is disclosed in J. Neurosci. Methods (Netherlands), V.86, No. 1, pp. 55-61 (1998), Malpeli J. G. The system operates by detecting the signal induced in a metal ring placed on the eye. This method is not a full-scale electromagnetic position and orientation tracker and, as described, is not distortion stable.
The need remains, therefore, for a simple but effective approach to reducing the effects of distortion in an electromagnetic tracking system. Ideally, such a solution would be useful in a variety of applications, including military, motion capture and medical instrumentation.
This invention provides a new technology and accompanying method for an electromagnetic position/orientation tracking in an environment wherein strong electromagnetic distortion may be present. In terms of apparatus, the system includes at least one source of an AC electromagnetic field, at least one witness sensor measuring components of the electromagnetic induction vector at known spatial points close to, or within the volume of interest, at least one wireless probe sensor placed on the object being tracked, and a control and processing unit.
The wireless sensor has a known response or distortion to the AC electromagnetic field generated by the primary source. The control/processing unit uses data from the witness sensor(s) to locate the probe sensor, treating the probe sensor as a secondary source of the AC electromagnetic field; that is, as transponder, a source with initially known magnetic parameters. This information is utilized by a position and orientation algorithm executed by the control/processing unit to define coordinates and attitude of the secondary source and, in turn the position and orientation of the object of interest.
In the preferred embodiment, the probe sensor is an LC-contour ring tuned to the frequency of the tracker source. As such, the signal from the probe sensor is 90xc2x0 phase shifted with respect to the signal from the tracker source, and, correspondingly phase shifted with respect to distortion. This allows the witness sensors and processor to separate the environmental distortion signal and the source signal from the probe sensor signal. The higher the Q, and the more accurate the tuning of the probe sensor, the higher the stability to the distortion.
In terms of methodology, initial measurements and computations are preferably performed before the known distorter/probe sensor has been introduced into the volume of interest, or at the frequency close but not equal to the resonant frequency of the probe sensor thereby providing a background profile of the field. The measurements are then repeated in real time, or quasi-real time in the presence of the probe sensor. The difference between initial and ongoing solution is completely determined by the position and orientation of the probe sensor working as a known distorter.
In addition to trackers for helmet-mounted displays in aircraft, tank, and armored-vehicle applications (FIG. 6), the invention finds utility in any electromagnetic tracking system which might be subject to electromagnetic distortion or interference. Such application areas include electromagnetic motion capture systems, and medical systems and instruments, among others.